These new clocks are already being applied for tests of fundamental physics and the possibility of a redefinition of the SI second in terms of an optical transition frequency is being considered by the international metrology community.
At NPL we are developing optical atomic clocks based on single trapped ytterbium and strontium ions, and a strontium optical lattice clock. To validate their uncertainty budgets, it is essential that we compare their frequencies to those of similar optical clocks being developed by independent research groups in other laboratories. These comparisons need to be performed at a level commensurate with the performance of the optical clocks. However, until recently, almost all information about optical clock reproducibility came from independent absolute frequency measurements made in different institutes, and was therefore limited by the uncertainty of the local caesium primary standards used to realise the SI second.
This situation is now changing. Optical clocks developed at NPL and in other European laboratories have recently been compared directly via GPS satellite links, by using a broadband two-way time and frequency transfer technique and over our recently established optical fibre link to LNE-SYRTE and PTB. Further information about clock reproducibility can be obtained by comparing optical frequency ratio measurements performed locally in different laboratories. Within the framework of the “International Timescales with Optical Clocks (ITOC)” project, new gravity surveys have also been carried out at and around NPL and other European National Measurement Institutes and used to compute an improved European geoid model. This has significantly improved our knowledge of the gravitational redshift corrections that must be applied when comparing optical atomic clocks in the different institutes and when using them in the future for international timekeeping.